This invention relates to a radiation-sensitive polymer capable of resistance to dry etching when it is applied to form very fine patterns in VLSI's and other semiconductor devices. The invention also relates to a radiation-sensitive composition containing said polymer.
While various microfabrication processes are known today that have been proposed to meet the growing demand for increasing the level of integration and packing density in semiconductor devices, a lithographic technique involving the formation of a multilayer film is under review (see FIGS. 1 and 2). This technique consists of forming a thick flat layer on a semiconductor substrate, then forming several thin layers containing a resist layer, and allowing the pattern in an upper layer to be transferred successively onto underlying layers by a suitable method such as dry etching. Conventionally, three or two layers are formed on the thick flat layer as shown in FIG. 2. A transfer method using a three-layer system is illustrated in FIG. 2 and consists of the following steps: forming a flat layer 5 on a semiconductor substrate 6; providing the flat layer 5 with an undercoat 4 that is resistant to a halogen gas plasma; providing the undercoat 4 with an intermediate layer 3 that is resistant to the oxygen plasma etching; forming a resist layer 2 on the intermediate layer 3; exposing the resist layer 2 imagewise through a mask 1; developing the resist layer 2 to form a pattern on it; transferring the resulting resist pattern 2a onto the intermediate layer 3 by plasma etching with a fluorine-containing gas; transferring the intermediate pattern 3a onto the undercoat 4 by the oxygen plasma etching; and further transferring the undercoat pattern 4a onto the flat layer 5 by plasma etching with a fluorine-containing gas. This process, however, has had the problem that several dry etching processes are necessary to transfer patterns and, therefore, the number of steps involved increases making the process complicated.
To overcome this problem, a single-layer resist process has been proposed which, as shown in FIG. 1, provides the flat thick layer 5 with a resist layer 7 that withstands dry etching. According to this process, the resist layer 7 is exposed through a mask 1 and developed to form a resist pattern 7a on the flat layer 5, with said pattern being transferred onto the flat layer by dry etching. Various compounds including chloromethylated polydiphenylsiloxane and chloromethylated polyphenylmethylsilane have been reviewed for use as resist materials that withstand the drying etching performed in this process. However, controlling the chloromethylated reaction is so difficult that these resist materials have a relatively broad molecular weight distribution due to the decrease in their molecular weight and low resolution will result in the use of such resist materials. Further, it is difficult to control the quality of the synthesized products since their characteristics are variable from lot to lot.
The resist material used in lithographic process by radiation exposure are classified into two groups, organics and inorganics. The term "radiation" as used herein covers all high-energy radiations including ultraviolet light, deep-ultraviolet light, electron beams and X-rays.
Organic resist materials, for example, polymethyl methacrylate (hereinafter abbreviated as "PMMA"), polyglycidyl methacrylate, polyhydroxystyrene, polyether sulfone and novolak resins, are used as binders and are available as a two-component system that also contains a suitable photo-sensitive compounds. A suitable acid generator may be added to this two-component system in order to cause chemical amplification and such a three-component system is also studied and be developed.
The organic resist materials have a good capacity to form homogeneous films, and high sensitivities at a specific wavelength allowing efficient development. However, most of the conventional organic resist materials are rather poor in resistance to the dry etching which is one of the steps involved in microfabrication processes. PMMA which has been known to be studied most extensively of the photo-sensitive resist materials today has the advantage of high resolution and low cost. However, PMMA is not highly resistant to dry etching or does not have high sensitivity. It has been attempted to improved the dry etching resistance by introducing a halogenated alkyl or phenyl group into the side chains (see Japanese Patent Public Disclosure Nos. 18638/1980, 254041/1985 and 234006/1988) but the modified PMMA is still unsatisfaction with regand to sensitivity and resolution. Polymers having benzene rings, such as polyhydroxystyrene, have also been studied to evaluate their lithographic properties for use as resist materials in microfabrication processes using high-energy radiations as a light source (see Japanese Patent Public Disclosure No. 446/1985). These polymers are yet to be used commercially for the sake of their lower resistance to the dry etching than that of novolak resins which are conventionally used in these applications.
Inorganic resist materials that have been studied and developed so far include amorphous chalcogenides and crystalline tungstenic acid. Inorganic resist materials show high resistance to oxygen gas plasma but give relatively heterogeneous coating films. For example, the formation of this films that utilize the high photo-sensitivity of amorphous chalcogenides (Japanese Patent Public Disclosure No. 27137/1981) requires a vacuum evaporation or sputtering, and the processing apparatus is expensive and yet achieves only a small throughput. In an attempt to solve these problems, crystalline condensed tungstenic acid that can be coated to form a film has been proposed (Japanese Patent Public Disclosure No. 100448/1988). This has improved sensitivity to radiations but achieves only limited resolution. Further, this is only compatible with a two-layered resist process.
Active efforts have also been made to develop resist materials that combine the advantages of organic resist materials with those of inorganics. For example, a photo-decomposable polymer such as PMMA which is an organic resist material has been mixed with an organometallic compound (see Japanese Patent Public Disclosure No. 202441/1983). However, this resist material has not yet been used commercially since it suffers the problem that an insoluble layer in a liquid developer forms on the resist surface during prebaking. Further, the organometallic compound is a key factor to providing resistance to the plasma etching due to miscibility problem, but there is only limited latitude in increasing the addition of this compound. Other problems associated with the blend system under consideration are low photo-sensitivity and low resolution.
As described above, no resist material has yet been attained that is suitable for use in a single-layered resist process of microfabrication on account of high radiation sensitivity, high resolution, high resistance to the dry etching and good film-forming property. The advent of such resist material has therefore been strongly desired.